This invention relates generally to media handling systems for inkjet printing devices, and more particularly to a media handling system which is capable of achieving small bottom margins.
Known inkjet printing devices which use single sheet or cut sheet media have a limited bottom margin capability. One of the smallest bottom margins achievable is approximately 11.7 mm by the Hewlett Packard 800 series DeskJet(trademark) printers. Many applications could take advantage of a smaller bottom margin, if available on single sheet and cut sheet inkjet printers. Continuous form inkjet products are able to achieve smaller bottom margins because a current page is attached to a subsequent page during printing. The pages are detached after printing.
Other inkjet printing concerns which impact the bottom margin limitation are the need for accurate dot placement and the need to account for the effects of wet ink printing. Both of these concerns impact a larger portion of the media sheet than simply the immediate area being printed at any given time. Media handling is one function controlled to achieve accurate printing and wet ink control. In the series 800 DeskJet(trademark) printers, for example, pinch rollers keep the media sheet in contact with a drive roller as the media sheet is fed through a print zone adjacent to a printhead. The pinch rollers prevent media slippage and allow for accurate dot placement. Cockle control devices such as ribbed devices place a known bend pattern in the paper downstream from the print zone which limits cockle growth in the print zone. The pinch rollers isolate the cockled area from a flat media sheet area in the print zone.
FIG. 1 shows a conventional inkjet printing apparatus 10 including an inkjet pen 12 having a printhead 14. The printhead 14 includes a plurality of inkjet nozzles which eject ink onto a media sheet 16 during printing. The media sheet is moved along a media path in a direction 17 by one or more rollers, including a drive roller 18. A pinch roller 20 presses the media sheet to the drive roller 18. A platen 22 supports the media sheet as the media sheet 16 is moved through a print zone 24. Typically, the print zone is located close to the pinch roller""s line of contact with the media sheet, but further along the media sheet path than the pinch roller""s line of contact. More specifically, the print zone 24 is located adjacent to the printhead nozzles between the printhead 14 and the platen 22. The relative location of the pinch rollers relative to the print zone determines how small the bottom margin can be. Once a trailing edge 26 of the media sheet 16 passes beyond the pinch roller 20, there is nothing securing the media sheet as the trailing edge 26 advances through the print zone 24. Accordingly, printing on the media sheet after contact is lost with the pinch roller 20 is subject to inaccuracies. The manufacturer avoids these inaccuracies by making the minimum bottom margin large enough that the media sheet is still in contact with the pinch roller at the bottom margin. Typically the distance from the nearest edge of the print zone to the pinch roller line of contact equals the minimum bottom page margin achievable for an inkjet print apparatus. Referring to FIG. 1, the minimum bottom page margin for the media sheet 16 is limited by the distance dp from the pinch roller 20 line of contact to the nozzle area of the printhead 14.
In a page wide array inkjet printhead the nozzle rows are oriented 90 degrees about the typical scanning printhead nozzle row orientation. In the FIG. 1 canning inkjet pen 12 the nozzle rows are oriented in a direction across the drawing sheet from let to right in the areas marked for the printhead 14. FIG. 2 shows a page wide array configuration. Inkjet pen 12xe2x80x2 includes a pagewide array printhead 14xe2x80x2 having a pair of rows 15, 19 for each color (e.g., YMCK-yellow, magenta, cyan and black). The rows 15, 19 are oriented to extend into the page of the drawing sheet.
Referring to FIG. 2, the media sheet 16 is moved along a media path in the direction 17 by one or more rollers, including a drive roller 18. A pinch roller 20 presses the media sheet to the drive roller 18. A platen 22 supports the media sheet as the media sheet 16 is moved through a print zone 24. Once a trailing edge 26 of the media sheet 16 passes beyond the pinch roller 20, there is nothing securing the media sheet as the trailing edge 26 advances through the print zone 24. Accordingly, the minimum bottom margin is made large enough that the media sheet is still in contact with the pinch roller. With the nozzles oriented in the direction parallel to the length of the pinch rollers and drive rollers, the minimum bottom margin is limited by the first row of nozzles located farthest from the pinch roller 20 (row 19 of color K in FIG. 2). Referring to FIG. 2, the minimum bottom page margin for the media sheet 16 is limited by the distance dpxe2x80x2 from the pinch roller 20 line of contact to the nozzle area of the printhead 14xe2x80x2. For a YMCK printhead, the minimum bottom margin would be greater than 1 inch.
One way of reducing the minimum bottom margin is to place the pinch roller 20 closer to the print zone 24. There is a limit, however, to how close the pinch roller line of media sheet contact can be to the print zone. Another scheme is to make the pinch roller diameter smaller, so that the distance between the print zone and pinch roller can be shorter. However, media advance accuracy suffers as the pinch roller becomes too small.
The pinch roller also serves to provide a reverse bowing which reduces cockle growth from the wet ink printing. Cockle growth refers to the buckling or ridges in a media sheet due to the presence of wet ink soaking into the media sheet. As the pinch roller becomes too small the reverse bow desired for limiting cockle growth becomes difficult to maintain. Accordingly, there is a need for a method and apparatus for allows for smaller bottom margins than the distance between pinch roller and print zone.
According to the invention, a media handling system provides accurate positioning of a media sheet through a print zone to allow for smaller bottom margins than known minimum bottom margins for single sheet or cut sheet inkjet printing devices. Minimum bottom margins less than 5 mm, and as low as 1 mm or 2 mm are achieved. This is a substantial improvement over the 11.7 mm minimum bottom margin achieved by current inkjet printing devices.
According to one aspect of the invention the media sheet is positioned on a support while traveling through the print zone. In addition to a pinch roller located along the media path prior to the print zone (i.e., an upstream pinch roller), there may be an another optional pinch roller located after the print zone (i.e., a downstream pinch roller). The support and pinch rollers stabilize the media sheet while the media sheet moves through the print zone. The downstream pinch roller may be of a star wheel configuration to minimize contact with the media sheet and avoid smudging the wet ink on the media sheet. A function of the downstream pinch roller is to hold the media sheet down and away from the inkjet printhead. Another function is to assist in advancing the media, especially once the media sheet trailing edge has passed beyond the upstream pinch roller.
According to another aspect of the invention, a guide shim is operatively positioned with the upstream pinch roller. The guide shim extends along the media path beyond the upstream pinch roller toward the print zone. The guide shim abuts or comes close to the print zone. The location of a lead edge of the guide shim relative to the print zone determines the minimum bottom margin for the inkjet printing device. One function of the guide shim is to provide media advance accuracy as the media sheet trailing edge departs contact with the upstream pinch roller and continues on to the print zone. Another function is to maintain the media flatness as the media sheet continues to the print zone. The guide shim serves to keep the media sheet under the inkjet printhead as the media sheet moves under the printhead. Cockle growth is limited by maintaining such flatness.
According to another aspect of this invention, the guide shim advances with the bottom edge of the media sheet into the print zone. As the guide shim is advanced, it keeps the media sheet in contact with the support, providing advance accuracy and minimal paper to pen spacing. The movement of the shim into the print zone also allows the minimum bottom margin to decrease. The minimum bottom margin is the distance from a prescribed location on the printhead to a portion under the distal edge of the advanced guide shim. For nozzle rows oriented perpendicular to the media path, the prescribed location on the printhead is the location of the nozzle row furthest from the guide shim. For nozzle rows oriented parallel to the media path, the prescribed location on the printhead is the location of the nozzles which are closest to the guide shim.
According to another aspect of the invention, the support is an endless belt loop driven by drive rollers. Preferably the belt has a ribbing or a grit coating. The media sheet rests on the belt and is stationary relative to the belt while moving through the print zone. The belt provides a continuous surface moving uniformly from the upstream pinch roller to the downstream pinch roller. The ribbing serves to reduce cockle growth due to the wet ink received on the media sheet. A grit coating, however, maintains more accurate referencing between the media sheet and the belt.
According to an alternative aspect of this invention, the support is a stationary platen which extends at least the length of the print zone. The media sheet is fed from the upstream pinch roller onto the platen, through the print zone and to the downstream pinch roller. The upstream pinch roller in combination with a drive roller and the downstream pinch roller in combination with another drive roller advance the media sheet.
According to an alternative aspect of this invention, the support is a moving platen which travels a path between the upstream pinch roller and downstream pinch roller through the print zone. The media sheet is fed from the upstream pinch roller onto the platen, through the print zone and to the downstream pinch roller. The upstream pinch roller in combination with a drive roller and the downstream pinch roller in combination with another drive roller advance the media sheet. The platen travels through the print zone with the trailing edge of the media sheet.
One advantage of the support, pinch roller, guide shim configuration is that media advance accuracy is maintained, and cockle growth is controlled, even while the media sheet trail edge leaves contact with the upstream pinch roller. A beneficial effect is that the minimum bottom margin is reduced. An advantage of the shim is that media advance accuracy is maintained even for pinch rollers which do not spin at identical speeds (e.g., due to manufacturing tolerances). These and other aspects and advantages of the invention will be better understood by reference to the following detailed description taken in conjunction with the accompanying drawings.